Carburizing method and apparatus



O. E. CULLEN Dec. 5, 1967 CARBURIZNG METHOD AND APPARATUS 4 sheets-sheet1 Filed Aug. 20, '1965 Dec. 5, 1967 o. E. CULLEN l CARBURIZING METHODAND'APARATUS 4 Sheets-Sheet. 5

Filed Aug. 2d, 1,965

FEV

INVENTOR.' URI/ILLE' E. EULLEN.

BY E @cw ATTYS.

Dec. 5, 1967 o. E. CULLEN 3,356,541

CARBURIZING METHOD AND APPARATUS Filed Ag. 20, 1965 4 Sheets-Sheet '4 ATTV1/s.

United States Patent O 3,356,541 CARBURIZING METHOD AND APPARATUSOrville E. Cullen, Toledo, Ohio, assgnor to Midland-Ross Corporation,Toledo, Ohio, a corporation of Ohio Filed Aug. 20, 1965, Ser. No.481,199 18 Claims. (Cl. 14S-16.5)

ABSTRACT F THE DHSCLOSURE The present invention relates to a continuousmethod for carburizing ferrous metal work. The work is passedsuccessively through longitudinally extending and aligned heating,carburizing and diffusing zones. In one embodiment of the invention ahigh carbon potential is maintained in the carburizing zone and a lowercarbon potential is maintained in the diffusing zone. The carbonpotential of the carburizing and of the diffusing zones is controllediby introducing into the heating zone a carrier gas incapable ofappreciable carburization of the work. Appreciable flow of the carriergas is prevented except in the direction of work movement through theheating zone. A mixture of a carrier gas and an enriching gas, inproportions to provide the carbon potential desired in the diffusingzone, is introduced into the diffusing zone at a rate sufficient tocau-se a flow of the gas mixture toward the carburizing zone in adirection opposed to the work travel. Atmosphere is withdrawn from thecarburizing zone at a rate which is substantially the sum of the rate atwhich the carrier gas is introduced into the heating zone and the rateat which the mixture of the carrier gas and the enriching gas is causedto flow toward the carburizing zone from the diffusing zone. Anenriching gas is introduced in the carburizing zone and mixed with theatmosphere therein to provide the desired carbon potential in thecarburizing zone. Substantially uniform and separate recirculation ofthe atmospheres within the carburizing and diffusing zones is effectedfrom a first to an opposed boundary of the zones and exteriorly of thezones :back to the first boundary. Concurrently, atmosphere from thecarburizing zone is prevented from flowing to either the heating zone orthe diffusing zone.

This invention relates to a continuous method for carburizing ferrousmetal Work.

In the art of carburizing it is known to use gaseous atmospheres toachieve carburization of ferrous metal work. United States Patent2,955,062 discloses a method for carburizing in a continuous furnacewherein the work passes through a carburizing zone of a high carbon p0-tential and then through .a diffusion zone having a lower car-bonpotential. The high carbon potential in the car burizing zone enablesthe introduction into the work, in a reasonable carburizing time, of anamount of carbon sufficient to provide a required case depth, while thelower carbon potential in the diffusion zone provides a desirable carboncontent adjacent the surface of the work. If carburization werecontinued at the high carbon potential of the carburizing zone, a carbondistribution generally as represented by FIG. 6 of the indicated patent,curve 1, would be the result. By virtue of the lowered carbon potentialin the diffusion zone a carbon distribution generally as represented inFIG. 6, curve 2, is achieved.

It has been found that the prior art method disclosed in U.S. Patent2,955,062 is very difficult to control. Present day specifications notonly specify the total case depth, which is established as a primarilyfunction of time and temperature, but also the specifications furtherspecify the character of the carburized case with respect to surfacecarbon, -preferred slope of the carbon gradient, and the depth ofeffective case measured to a desired carbon level or to a specifiedRockwell hardness, all of which are highly dependent upon the carbonpotential of the surrounding atmosphere.

It has been found that where undesired spillover occurs between theatmospheres of `adjacent zones it is extremely difficult to consistentlymeet present day specifications.

It is the primary object of the instant invention to provide an improvedmethod and apparatus for carburizing in a continuous furnace.

It is a still further object of the instant invention to provide animproved method and apparatus for carburizing ferrous metal work wherebythe work is case hardened `accurately with respect to surface carbon,slope of carbon gradient curve and dept-h of effective case.

It is still another object of the instant invention to provide animproved method for carburizing ferrous metal work wherein a high degreeof control is achieved and wherein the work is processed in a shortenedperiod of time.

Further objects of this invention will become apparent from thefollowing specication and drawings in which:

FIG. l is a vertical sectional view in elevation, with parts removed forclarity, showing a continuous furnace used in practicing the presentinvention;

FIG. 2 is a vertical sectional view shown on an enlarged scale, andtaken along the line 2-2 of FlG.l 1;

FIG. 3 is a vertical sectional view, shown on an enlarged scale, andtaken alongthe line 3 3 of FIG. 1;

FIG. 4 is a diagrammatic plan View, partially shown in section, of thefurnace of FIG. l;

FIG. 5 is a graph showing atmosphere carbon potential throughout thefurnace of FIG. 1 while practicing one embodiment of the presentinvention and also showing atmosphere carbon potential of a prior artmethod; and

FIG. 6 is a diagrammatic view, similar to FIG. 4 and illustratinganother embodiment of the present invention.

Briefly, the present invention rela-tes to a continuous method forcarburizing ferrous metal work. The work is passed successively throughlongitudinally extending and aligned heating, carburizing and diffusingzones. In

one embodiment of the invention a high carbon potentialV is maintainedin the carburizing zone and a lower carbon potential is maintained inthe diffusing zone. The carbon potential of the carburizing and of thediffusing zones is controlled by introducing into the heating zone acarrier gas incapable of appreciable carburization of the work.Appreciable flow of the carrier gas is prevented except in the directionof work movement through the heating zone. A mixture of a carrier gasand an enriching gas, in proportions to provide the carbon potentialdesired in the diffusing zone, is introduced into the diffusing zone ata rate sufficient to cause a flow of the gas mixture toward thecarburizing zone in a direction opposed to the work travel. Atmosphereis withdrawn from the carburizing zone at a rate which is substantiallythe sum of the rate at which the carrier gas is introduced into theheating zone and the rate at which the mixture of the carrier gas andthe enriching gas is caused to flow toward the carburizing zone from thediffusing zone. An enriching gas is introduced in the carburizing zoneand mixed with the atmosphere therein to provide the desired carbonpotential in the carburizing zone. Substantially uniform and separaterecirculation of the atmospheres within the carburizing and diffusingzones is effected from a first to an opposed boundary of the zones andexteriorly of the zones back to the first boundary. Concurrently,atmosphere from the carburizing zone is prevented from flowing to eitherthe heating zone or the diffusing zone.

In another embodiment, according to the present invention a carrier gasincapable of appreciable carburization of the work is introduced intothe heating zone.

Appreciable flow of the carrier gas is prevented except in the directionof work movement through the heating zone. An enriching gas isintroduced in the carburizing zone to provide the desired carbonpotential in the diffusing zone. Atmosphere is withdrawn from thediffusing zone at a rate sufiicient to cause a flow of atmosphere fromthe carburizing zone. An oxidizing gas is introduced in the diffusionzone and mixed with the atmosphere therein to provide the desired carbonpotential in the carburizing zone. Atmosphere is withdrawn from thediffusing zone at substantially the rate at which atmosphere isintroduced into the several zones.

Carbon potential of a fiuid, as used herein and in the appended claims,indicates the carbon content to which that gas will carburize steel ifequilibrium is reached. It is customarily measured in percent of carbonin thin strips of steel which have been brought to substantialequilibrium with the gaseous atmosphere and have a substantially uniformcarbon content throughout the strip.

Carbon potential is also a function of temperature. At least within theaustenitic range the carbon potential of a gas of a given compositionincreases with decreases in temperature. Known gases having a carbonpotential within the range usually required for conventionalcarburization, namely, 0.60 to 1.40 percent, at normal carburizingtemperatures, decompose badly and deposit large quantities of soo-t atlower temperatures. Lower decomposition temperatures are present withinthe temperature range to which work is subjected during the initialheating step and often during a cooling step. Therefore, it is importantthat the high carbon potential atmosphere of the carburizing zone doesnot flow out of the carburizing zone into the adjacent heating zonewhich has regions which are at decomposing temperatures, but such fiowto the diffusing zone is not necessarily harmful.

The term enriching gas, as used herein, means a CH.,l gas, which termincludes natural gas, relatively pure methane, ethane, propane, andother hydrocarbons and oxyhydrocarbons that are methane equivalents inthat they are known enriching gases for c-ar-burizing.

The term carrier gas, as used herein, refers to a gas having thefollowing volume composition: 12 to 25% CO, to 50% H2, traces of CH4,H2O, and CO2, and the balance of at least N2. Preferably, a typicalcarrier gas which could be enriched with methane comprises, for example:

Percent by volume CO2 Trace CO 20.7 H2 38.7 CH., 0.8 H2O 1Trace N2Balance 1 Dew point 20 F.

The term oxidizing gas, as used herein, refers to a gas such as air,H2O, or CO2 and also includes one or more of these gases mixed with asubstantially neutral gas, e.g. a carrier gas.

Referring to FIG. l of the drawings, a heat treating furnace is`generally indicated at 10. The furnace 10 comprises a metallic casingor shell 11 which encloses a refractory structure 12. An inlet opening13 and a discharge opening 14 are located at opposed ends 15 and 16 ofthe furnace 10. The openings 13 and 14 are in communication withvestibules (not shown) having doors which are electrically interlocked,as is well known in the art, to prevent loss of the respective furnaceatmospheres.

In addition to the ends 15 and 16, the furnace 10 has a top 17, a bottom18 and opposed side Walls 19 and 20. Pairs of solid piers 21 (FIG. 3)extend upwardly from the bottom 18 at a location spaced longitudinallyfrom the end 15 of the furnace 10 and support a pair of arches 22.Similarly, pairs of piers 23 are spaced from the opl posite end 16 ofthe furnace 10 and support arches 24.

The end wall 15, the top wall 17, the bottom Wall 18, the side walls 19and 20, and the pair of arches 22 define a heating zone 25 of thefurnace 10. The arches 22, the arches 24, the top wall 17, the bottomwall 18, and the side walls 19 and 20, define a carburizing zone 26 ofthe furnace 10. Similarly, the arches 24, the end Wall 16, the top wall17, the bottom 18, and the side Walls 19 and 20 define a diffusing zone27 of the furnace 10.

It should be expressly understood that other heat treatment operationsmay be performed upon the work before it enters the heating zone 25 andsubsequent to its discharge from the diffusing zone 27. For example, aquench operation subsequent to the discharge from the diffusing zone 27is common, and the present invention is not limited by preliminaryoperations or subsequent operations, but is directed to a method ofcarburizing which may be but one operation in the treatment of ferrouswork. A plurality of intermediate walls 28 and 28a extend transverselybetween the side walls 19 and 20 and also extend upwardly from thebottom 18 of the furnace 10. Preferably, lower radiant tubes 29 arepositioned between adjacent intermediate walls 28 and 28a and serve asheating means. The intermediate walls 28 are of checkered construction,and are in the heating zone, while the walls 28a between the respectivepairs of piers 21 and 23 in the carburizing and diffusing zones aresolid. The walls 28a are effective to prevent atmosphere movementbetween the respective furnace zones. The lower radiant tubes 29 may beeither gas fired or electrical radiant tubes and preferably are eitherindividually or zone controlled with respect to their temperatures.Similarly, upper radiant tubes 30 are longitudinally spaced adjacent thetop 17 of the furnace 10.

Conveyor means 31 are supported by the intermediate Walls 28 and 28a,and by the respective end Walls 15 and 16. The conveyor means 31 may beof any of the well known types, for example, a series of transverselyextending rollers or in the alternative longitudinally extending tracks.The conveyor means 31 support a plurality of work pallets, indicated at32 in FIGS. 2 and 3. Ferrous work to be treated, for example gears, isplaced on the pallets 32 which are moved along the conveyor means 31.The conveyor means 31 includes driving means, for example, a chainconveyor or a pusher (not shown), suitable for moving the pallets 32through the furnace 10 in a conventional manner.

Recirculating means are provided both in the carburizing zone 26 and inthe diffusing zone 27. In the instant embodiment, the recirculatingmeans comprises side fans 33 and 34 which are mounted adjacent the sidewall 19 of the furnace 10 in the carburizing zone 26 and in thediffusing zone 27, respectively.

The recirculating rates within the carburizing zone 26 and the diffusingzone 27 are an important feature of the present invention. It has beenfound that the recirculating rates should preferably be at least 50times the rate of the atmosphere entering the furnace 10.

The high recirculating rates, insure a quick recovery of the desiredzone atmospheres if they are lost during furnace operation. It has beenfound that atmosphere recovery takes only a matter of minutes comparedto prior art recovery rates which were often extremely lengthy.

Referring to FIG. 2, the side fan 33 is driven by a motor which ismounted adjacent the shell 11 of the furnace 10. The side wall 19 of thefurnace 10 defines an intake opening 35 which communicates with arecirculating passageway 36. The recirculating passageway 36 is definedby spaced portions of the side wall 19 and is generally Vertical. Theside wall 19 also defines discharge openings 37 near the bottom 18 whichcommunicate with the recirculating passageway 36 and the intake Opening35. Preferably, the side fan 33 withdraws atmosphere at a given ratefrom the carburizing zone 26 through the intake opening 35 downwardlythrough the recirculating passageway 36 and discharges the recirculateduid through the discharge openings 37 at a position below the conveyormeans 31. It has been found that recirculating iiuid in this mannerproduces a plenum effect in the area below the conveyor means 31. It hasalso been found that this plenum effect is extremely useful in thecontrolling of the carburizing atmosphere within the carburizing zone26. The results achieved by utilizing this plenum effect are farsuperior to the effects achieved by merely recirculating with, forexample, overhead fans.

In like manner, the side wall 19 has an intake opening 38 adjacent theside fan 34 located in the diffusing zone 27. The opening 38communicates with a generally vertical recirculating passageway 39(indicated by dashed lines in FIG. l) and the discharge openings 40 arelocated in the Wall 19 near the bottom 18.

An etliuent opening 41 (see FIG. 2) is provided in the side wall 20 ofthe furnace 10. The effluent opening 41 is in communication with thecarburizing zone 26 and atmosphere is withdrawn from the carburizingzone 26, for example through an orifice (not illustrated).

In a typical Work carburizing operation, according to the firstembodiment of the present invention, Work is placed on the trays orpallets 32 and is introduced into the heating zone 25 through the inletopening 13. The work then passes successively through the longitudinallyextending heating zone 25, the carburizing zone 26, and the diffusingzone 27, and outwardly through the discharge opening 14 of the furnace10.

As the metal work passes through the heating zone 25 its temperature israised from its entrance temperature to a carburizing temperature ofbetween l350 F. to l800 F. Normally, the carburizing temperature isapproximately 1700 F. The carbon content of the ferrous metalV Work tobe carburized normally falls between 0.10% and 0.60%, when it isintroduced into the heating zone 25. A carrier gas, of the compositiondescribed above, is introduced into the heating zone by suitable piping(not shown) at a predetermined rate. The carrier gas may have anappreciable carbon potential, but is incapable of appreciablecarburization of the ferrous metal work. If the carbon potential of thecarrier gas is too high or if the atmosphere of the carburizing zone 26flows into the heating zone 25, there is a tendency to deposit largeamounts of soot at the relatively low temperature regions of the heatingzone.

After the ferrous metal work is heated to its proper carburizingtemperature, it passes into the carburizing zone 26. An enriching gas isintroduced into the carburizing zone 26 through suitable piping (notshown) at a sufficient rate to insure a high carbon potential atmospherein the carburizing zone 26. The carbon potential of the carburizing zoneatmosphere is normally held at between 1.0% and 1.4%.

The ferrous metal work then vpasses through the carburizing zone intothe diffusing zone 27. A mixture of a carrier gas and an enriching gasin proportions to provide the predetermined carbon potential desired inthe diffusing zone atmosphere is introduced into the diffusing zone 27through suitable piping (not shown).

An important feature of the present embodiment of the method is that aportion of the carburizing zone atmosphere is withdrawn through theeflluent opening 41 to draw atmosphere from the heating zone 25 anddiffusing zone 27 into the carburizing zone 26. Such atmosphere flow, incooperation with the vigorous atmosphere circulation, as discussedabove, yeffectively prevents reverse flow, or flow from the carburizingzone 26 into the heating zone 25 or into the diffusing zone 27. Thedischarge of atmosphere from the carburizing zone 26, through theeffluent piping 42, is at a rate which is substantially-the sum of therates at which atmosphere is introduced into the several zones of thefurnace.

It has been found that the present method gives consistent andreproducible results in the carburizing yof ferrous metal work. Superiorresults are achieved by the accurate controlling of the atmospheres inthe various zones and by the elimination of atmosphere ow from thecarburizing zone 26 into the adjacent zones 25 and 27.

FIG. 5 is a graph showing atmosphere carbon potential in relationship tovarious longitudinal locations Within the various furnace zones 25, 26,and 27. It should be noted that curves A and B are both ideal curves.Curve A represents the atmosphere carbon potential through the furnacewhen practicing the method disclosed in U.S. Patent 2,955,062 and curveB represents the atmosphere carbon potential through the furnace whenpracticing the present method.

In the prior art method disclosed in U.S. Patent 2,955,- 062, a majorportion of the total furnace atmosphere Was introduced into thecarburizing zone and a gentle atmosphere ow was maintained from thecarburizing zone into the adjacent zones. Ideally, a dynamic equilibriumwould be established within the furnace, by chemical reaction betweenthe Work and the atmosphere in the heating zone and by controlledaddition of an oxidizing gas to the diffusing zone. In practice, thedynamic equilibrium was difficult to achieve at all, and impossible tomaintain. For example, ow of atmosphere to the heating zone at a rategreater than that instantaneously required for equilibrium would causeexcessive sooting, and any change in the rate of ow of atmosphere to theheating zone would necessitate a compensating change in the rate atwhich atmosphere was introduced into the carbu- Yrizing zone, thediffusing zone, or both.

Similarly, in the prior art method, the carbon potential of thediffusing zone atmosphere decreased gradually from a high point whichcorresponded to the carbon potential of the carburizing zone atmosphereto a 10W carbon potential, which was the desired final carbon content ofthe surface of the ferrous metal work which Was being carburized.Finally, because the method depended upon establishing a dynamicequilibrium with slow moving gases, a long period of time was requiredto reestablish the equilibrium after any disruption thereof, forexample, by excessive opening of the furnace door in a given period oftime.

Under the method of the present invention, the carbon potentials of theatmospheres in the heating zone 25 and the diffusing zone 27 remainsubstantially on a horizontal v path throughout the lengths of therespective zones. Several advantages are achieved because of the uniquecarbon potential curve B achieved by the present method, as illustratedin FIG. 5. First, the carbon potentials of the atmospheres in therespective zones can be controlled closely. All three of the zones canbe regulated independen tly because of the unidirectional flow ofatmosphere between adjoining zones. Therefore, the carbon potentials ofthe heating zone atmosphere and of the diffusing zone atmosphere may bereadily observed and adjusted, if necessary. Under the present method itis possible to carburize ferrous metal work in strict accordance with aset of rigid specifications. In other words, surface case depth, thelength and slope of the carbon gradient, and the total case depth may bevery accurately controlled by using the present method of carburizing.

Another advantage is that time savings of between 10% and 25% areachieved when using the instant method. The time savings result from theaccurate atmosphere control which is illustrated by curve B in FIG. 5,as distinguished from curve A.

As the ferrous metal work passes longitudinally through the respectivezones, the atmospheres are recirculated within the carburizing zone 26and within the diffusing zone 27. Referring to FIGS. l and 2, in thecarburizing zone 27 recirculated fluid is discharged through thedischarge openings 37 beneath the conveyor means 31. As was previouslymentioned, the conveyor means 31 and the work pallets 32 tend topartially restrict tbe upward ow of fiuid and a plenum effect results.The fluid rises in the carburizing zone 26 from a first boundary of thezone, which is adjacent the bottom 18, upwardly, and uniformly contactsthe ferrous metal work which is being carried by the pallets 32. Thecarburizing Huid continues upwardly until it reaches an opposed boundaryof the carburizing zone 26 which is adjacent the top 17 of the furnace10. A portion of the fluid is disspelled outwardly at a predeterminedrate, as described above, through the effiuent opening 41 and throughthe efflulent piping 42. The remainder of the fluid passes through theintake opening 3S, downwardly through the recirculating passageway 36which is located exteriorly of the carburizing zone 26 and isrecirculated through the discharge openings 37.

The atmosphere within the diffusing zone 27 recirculates in a similarmanner. However, a predetermined portion of the diffusing zoneatmosphere is caused to ow toward and into the carburizing zone. Theremainder of the fiuid is recirculated through the recirculatingpassageway 39 (see FIG. 1). It should be noted that while the apparatusdisclosed above provides recirculation along a path within the zones 26and 27 from a bottom boundary to a top boundary, other apparatus whichestablishes, for example, a transverse path may be used in practicingthe present method.

A second.embodiment of the present invention, is shown in FIGURE 6.

In this embodiment, an effluent opening 43 is provided in the sidewall20a of the furnace 10a. The effluent opening 43 is in communication withthe diffusing zone 27a.

A carrier gas, of the composition described, is introduced into theheating zone 25a.

An enriching gas is introduced into the carburizing zone 26a at asufficient rate to insure a high carbon potential atmosphere in thecarburizing zone 26a. However, in this embodiment a portion of theatmosphere of the diffusing zone 27a is withdrawn through the efhuentopening 43 whereby atmosphere flows into the diffusing zone 27a from thecarburizing zone 26a.

In this embodiment reverse flow from the carburizing zone 26a to theheating zone 25a and from the diffusing zone 27a to the carburizing zone26a is prevented. Rather, a controlled ow in the direction of worktravel is induced.

An oxidizing gas is introduced into the diffusing zone 27a to maintainin the diffusing zone an atmosphere having a composition substantiallydifferent than that of the atmosphere in the carburizing zone.

In general, the embodiment of the invention illustrated in FIG. 6 ispreferred when it is desired to operate the diffusing zone at atemperature substantially lower than that of the carburizing zone, forexample, to prevent distortion of the carburized work during asubsequent quenching operation. For example, the carburizing zone may beoperated at a temperature of about 1700 F., while the diffusing zone isoperated at a temperature of about 1550 F., and it might be desired tomaintain the atmosphere of the carburizing zone at a carbon potential ofabout 1.05 and the atmosphere of the diffusing zone at a carbonpotential of 0.90. Appropriate atmospheres derived from a carrier gasproduced from methane and enriched with methane would have the followingapproximate compositions:

Carburizing Zone Diftusing Zone 0. 08 0. 30 19. 2 20. 5 40. 4 39. 2 1.6 1. 0 Balance Balance contents of the atmospheres, but the critical andsubstantial difference in carbon dioxide content and dew point would besubstantially the same.

The embodiment of the invention shown in FIG. 4 is usually preferredwhere a greater carbon potential differential is desired between thecarburizing zone and the diffusing zone atmospheres. In such cases thetwo zones are usually operated at approximately the same temperature,for example about 1700 F. In a specific instance it may be desired tomaintain the atmosphere of the carburizing zone at a carbon potential of1.20 and the atmosphere of the diffusing zone at a carbon potential of0.90. Using a carrier gas derived from methane and methane as anenriching gas, the following atmospheres would be appropriate:

Carburizing Zone Diffusing Zone CO2 O. 065 0. 095 CO 18.5 19.8 HzA 41. 54i). 2 CHL.-4 2. 2 0. 8

N z Balance B alance Dew Point, F 7 14 While the present invention hasbeen disclosed in connection with a specie arrangement of parts, it isto be expressly understood that numerous modifications and changes maybe made without departing from the scope of the appended claims.

I claim:

1. In a continuous method for carburizing ferrous metal work whichincludes the steps of passing the work successively throughlongitudinally extending and aligned heating, carburizing, and diffusingzones while maintaining a temperature between 1350 F. and 1800 F. ineach of the zones, the improvement of controlling the carbon potentialof the carburizing zone between 0.6 and 1.4 percent and diffusing zonebetween 0.6 and 1.1 percent by causing atmosphere flow in one directionwhile preventing such flow in the opposite direction between the two,introducing into the heating zone a carrier gas incapable of appreciablecarburization of the work, preventing appreciable ow of the carrier gasexcept in the direction of work movement through the heating zone andinto the carburizing zone, mixing with the atmosphere which flows intothe carburizing zone from at least one of the other zones an enrichinggas in a proportion to provide the carbon potential desired therein,preventing appreciable ow of atmosphere from the carburizing zone to theheating zone, introducing into the diffusing zone an atmosphere which,when mixed with any atmosphere flowing into the diffusing zone from thecarburizing zone, provides the atmosphere required therein, andwithdrawing atmosphere from the one of the carburizing and diffusingzones toward which atmosphere ows from the other at substantially therate at which atmosphere is introduced into the several zones.

2. In a continuous method for carburizing ferrous metal work whichincludes the steps of passing the work successively throughlongitudinally extending and aligned heating, carburizing, and diffusingzones while maintaining a temperature between l350 F. and 1800 P. and ahigh carbon potential in the carburizing zone and similar temperaturerange but a lower carbon potential in the diffusing zone, theimprovement of controlling the carbon potential of the carburizing zonebetween 0.6 to 1.4 percent and of the diffusing zone between 0.6 to 1.1

percent by introducing into the heating zone a carrier gas incapable ofappreciable carburization of the work, preventing appreciable fiow ofthe carrier gas except in the direction of work movement through theheating zone, introducing into the diffusing zone a mixture of a carrierfgas and an enriching gas in proportions to provide the carbon potentialdesired in the diffusing zone, and at a rate sufficient to cause a ow ofthe gas mixture toward the carburizing zone, withdrawing atmosphere fromthe carburizing zone at a rate which is substantially the sum of therate at which the carrier gas is introduced into the heating zone andthe rate at which the mixture of the carrier gas and of the enrichinggas is caused to flow toward the carburizing zone, mixing with theatmosphere in the carburizing zone an enriching gas in a proportion toprovide the carbon potential desired therein, and preventing appreciableflow of atmosphere from the carburizing zone to either of the heatingand diffusing zones.

3. In a continuous method for carburizing ferrous metal'work whichincludes the steps of passin-g the work successively throughlongitudinally extending and aligned heating, carburizing, and diffusingzones while maintaining a temperature between l350 F. and l800 F. and a,carbon potential between 0.16 and 1.4 percent in the carburizing zoneand a similar temperature range but a lower carbon potential in thedifusing zone, the improvement of controlling the carbon potential ofthe carburizing and of the diffusing zones by introducing into theheating zone a carrier gas incapable of appreciable carburization of thework, maintaining the heating zone at a temperature approximating thetemperature of the carburizing zone, preventing appreciable ilow of thecarrier gas except in the direction of work movement through the heatingzone, and into the carburizing zone, introducing into the diiusing zonea mixture of a carrier gas and an enriching gas in proportions toprovide the carbon potential desired in the diffusing zone, and at arate sufficient to cause a flow of the gas mixture toward thecarburizing zone, withdrawing atmosphere from the carburizing zone at arate which is substantially the sum of the rate at which the carrier gasis introducedinto the heating zone and the rate at which the mixture ofthe carrier gas and of the enriching gas is caused to flow toward thecarburizing zone to induce a flow of atmosphere thereto Vfrom theheating and diffusing zones, mixing with the atmosphere in thecarburizing zone an enriching gas in a proportion to provide the carbonpotential desired therein, and preventing appreciable flow of atmospherefrom the carburizing zone to either of the heating and diffusing zonesby circulating atmosphere at a high rate, relative to the rate at whichatmosphere is supplied to the several zones in a direction which hassubstantially no component longitudinal of the zones from a rst boundaryto an opposed boundary of each of the carburizing and diffusing zones,and from each opposed boundary, exteriorly of the zone, back to therespective rst boundary.

4. The improvement as claimed in claim 3l wherein the rates ofatmosphere circulation of atmosphere within the carburizing anddiffusing zones are at least about 100 times the rate at whichatmosphere is supplied to the several zones.

5. The improvement as claimed in claim 4 wherein the circulation ofatmosphere within the carburizing and diffusing zones is from plenumsadjacent the iirst boundaries thereof, through the zones and back to theplenums.

6. The improvement as claimed in claim 5 wherein the plenums areimmediately below the respective zones.

7. In a continuous method for carburizing ferrous metal work whichincludes the steps of passing the work successively throughlongitudinally extending and ali-gned heating, carburizing, anddiffusing zones while maintaining a temperature between 1350" F. and1800* F. in each of the zones, the improvement of controlling the carbonpotential of the carburizing zone between 0.6 to

1.4 percent and of the diffusing zone between 0.6 and 1.1 percent byintroducing into the heating zone a carrier gas incapable of appreciablecarburization of the work, preventing appreciable flow of the carriergas except in the direction of work movement through the heating zoneand into the carburizing zone, introducing into the carburizing zone anenriching gas in the proportion required to establish and maintain thecarbon potential desired in the carburizing zone, withdrawing atmospherefrom the diffusing zone at a rate sufficient to induce a ow ofatmosphere from the carburizing zone toward the diffusing zone, mixingwith the atmosphere in the diffusing zone an oxidizing gas and anycarrier gas that may be required in proportions to provide the carbonpotential desired therein, and preventing appreciable flow of atmospherefrom the carburizing zone to the heating zone and from the difusing zoneto the carburizing zone by circulating atmosphere at a high rate,relative to the rate at which atmosphere is supplied to the severalzones, in a direction which has substantially no component longitudinalof the zones from a first boundary to an opposed boundary of each of thecarburizing and diffusing zones, and from each opposed boundary,exteriorly of the Zone, back to the respective first boundary.

8. The improvement as claimed in claim 7 wherein the rates of atmospherecirculation of atmosphere within the carburizing and diffusing zones areat least about times the rate at which atmosphere is supplied to theseveral zones.

9. The improvement as claimed in claim wherein the circulation ofatmosphere within the carburizing and dif- `fusing zones is from plenumsadjacent the iirst boundaries thereof, through the zones and back to theplenums.

10. The improvement Ias claimed in claim 9 wherein the plenums areimmediately below the respective zones.

11. In a continuous method for carburizing ferrous metal work, the stepscomprising: introducing the work into a heating zone at a temperaturebetween 1350" F. and 1800 F., providing the heating zone with a firstgas, allowing the work to attain a temperaure beween 1350 F. and l800F., passing the work into a carburizing zone at a temperature `between1350 F. and 1800 F., preventing appreciable ow of the first gas exceptin the direction of work movement through the heating Zone and into thecarburizing zone, mixing the I'irst gas which iiows into the carburizingZone with a second gas, the composition of the gases being such that theresulting mixture has a carbon potential between 0.6 and 1.4 percent,preventing appreciable flow of atmosphere from the carburizing zone tothe heating Zone, allowing the Work to remain within the carburizingzone until the work achieves a carbon potential between 0.6 and 1.4percent, conveying the work into a diffusing zone, introducing into thediffusing zone an atmosphere which has a carbon potential of less thanthe mixture in the carburizing Zone, preventing appreciable flow of thediiusing atmosphere except in a direction opposed to work movement, andwithdrawing atmosphere from the carburizing zone at a rate at whichatmosphere is introduced into the several zones.

12. The method of claim 11 including the step of circulating atmosphereat a high rate, relative to the rate at which atmosphere is supplied tosaid several zones, in a direction which has substantially no componentlongitudinal of said furnace from a first boundary to an opposedboundary of each of the carburizing and diiiusing zones, and from eachopposed boundary, exteriorly of said zone, back to the respective firstboundary.

13. In a continuous method for carburizing ferrous metal work` theVsteps comprising: introducing the work into a heating zone at atemperature between 1350 F. and 1800 F., providing the heating zone witha first gas, allowing the work to attain a temperature between 1350 F.and 1800 F., passing the work into a carburizing zone at a temperaturebetween 1350 F. and 1800 F., preventing appreciable ow of the first gasexcept in the direction of work movement through the heating zone andinto the carburizing zone, mixing the first gas which flows into thecarburizing zone with a second gas, the composition of the gases beingsuch that the resulting mixture has a carbon potential between 0.6 and1.4 percent, preventing appreciable iiow of atmosphere from thecarburizing zone to the heating zone, allowing the Work to remain withinthe carburizing zone until the work achieves a carbon potential between0.6 and 1.4 percent, conveying the work into a diffusing zone, allowinga portion of the carburizing atmosphere to flow into the diffusing zone,introducing into the diffusing zone a third gas which, when mixed withany atmosphere flowing into the diffusing zone from the carburizingzone, provides an atmosphere having a carbon potential of less than 1.1percent, and withdrawing atmosphere from the diffusing zone at a rate atwhich atmosphere is introducing into the several zones.

14. The method of claim 13 including the step of circulating atmosphereat a high rate, relative to the rate at which atmosphere is supplied tosaid several zones, in a direction which has substantially no componentlongitudinal of said furnace from a first boundary to an opposedboundary of each of the carburizing and diffusing zones, and from eachopposed boundary, exteriorly of said zone, back to the respective firstboundary.

15. Apparatus for continuously carburizing Vferrous metal workcomprising: a furnace including longitudinally extending and alignedheating, carburizing, and diffusing zones, means for conveying worksuccessively through said zones, means for heating work within saidzones, means for introducing into said heating zone a carrier gasincapable of appreciable carburization of the work, means forintroducing into said carburizing zone a mixture of a carrier gas and anenriching gas in proportions to establish and maintain the carbonpotential between 0.6 to 1.4 percent in the carburizing zone, means forwithdrawing atmosphere from said diffusing zone at a rate sufficient toinduce a flow of atmosphere from said heating zone toward saidcarburizing zone and from said car-burizing zone toward said diffusingzone, means for mixing with the atmosphere in said diffusing zone anoxidizing gas and any carrier gas that may 'be required in proportionsto provide a carbon potential between 0.6 and 1.1 percent therein, andmeans for preventing appreciable flow of atmosphere from saidcarburizing zone to said heating zone and from said diffusing Zone tosaid carburizing zone.

16. The apparatus of claim 15 wherein said iiow preventing meansincludes means for circulating atmosphere at a high rate, relative tothe rate at which atmosphere is supplied to said several zones, in adirection which has substantially no component longitudinal of saidfurnace from a first boundary to an opposed boundary of each of saidcarburizing and diffusing zones, and from each opposed boundary,exteriorly of said zone, back to the rcspective first boundary.

17. Apparatus for continuously carburizing ferrous metal workcomprising: a furnace including longitudinally extending and alignedheating, carburizing, and diffusing zones, means for conveying worksuccessively through said zones, means for heating work within saidzones, means for introducing into said heating zone a carrier gasincapable of appreciable carburization of the work, means forintroducing into said diffusing zone a mixture of a carrier lgas and anenriching gas in proportions to provide a carbon potential between 0.6to 1.4 percent in said diffusing zone, means yfor withdrawing atmospherefrom said carburizing zone to induce a flow of atmosphere thereto fromsaid heating and diffusing zones, means for mixing with the atmospherein said carburizing zone an enriching gas in a proportion to provide acarbon potential between 0.6 and 1.1 percent therein, and means forpreventing appreciable fiow of atmosphere from said carburizing zone toeither of said heating and diffusing zones.

18. The apparatus of claim 17 wherein said ow preventing means includesmeans for circulating atmosphere at a high rate, relative to the rate atwhich atmosphere iS supplied to said several zones, in a direction whichhas substantially no component longitudinal of said furnace from a firstboundary to an opposed boundary of each of said carburizing anddiffusing zones, and from each opposed boundary, exteriorly of saidzone, back to the respective first boundary.

References Cited UNITED STATES PATENTS 2,955,062 10/1960 Cullen et al14S-16.5 3,218,323 4/1964 Davis 14S-16.5 X 3,189,336 6/l965 Montagino14S-16.5 X

CHARLES N. LOVELL, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,356,541 December S, 1967 Orville E. Cullen It is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column l, line 68, "primarily" should read primary Column 7, line l0,"efflulent" should read effluent Column 8, line 34, "specie" should readspecific Column ll, line 17, "introducing" should read introduced Signedand sealed this 19th day of August 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

1. IN A CONTINUOUS METHOD FOR CARBURIZING FERROUS METAL WORK WHICHINCLUDES THE STEPS OF PASSING THE WORK SUCCESSIVELY THROUGHLONGITUDINALLY EXTENDING AND ALIGNED HEATING, CARBURIZING, AND DIFFUSINGZONES WHILE MAINTAINING A TEMPERATURE BETWEEN 1350* F. AND 1800* F. INEACH OF THE ZONES, THE IMPROVEMENT OF CONTROLLING THE CARBON POTENTIALOF THE CARBURIZING ZONE BETWEEN 0.6 AND 1.4 PERCENT AND DIFUSSING ZONEBETWEEN 0.6 AND 1.1 PERCENT BY CAUSING ATMOSPHERE FLOW IN ONE DIRECTIONWHILE PREVENTING SUCH FLOW IN THE OPPOSITE DIRECTION BETWEEN THE TWO,INTRODUCING INTO THE HEATING ZONE A CARRIER GAS INCAPABLE OF APPRECIABLECARBURIZATION OF THEWORK, PREVENTING APPRECIABLE FLOW OF THE CARRIER GASEXCEPT IN THE DIRECTION OF WORK MOVEMENT THROUGH THE HEATING ZONE ANDINTO THE CARBURIZING ZONE, MIXING WITH THE ATMOSPHERE WHICH FLOWS INTOTHE CARBURIZING ZONE FROM AT LEAST ONE OF THE OTHER ZONES AN ENRICHINGGAS IN A PROPORTION TO PROVIDE THE CARBON POTENTIAL DESIRED THEREIN,PREVENTING APPRECIABLE FLOW OF ATMOSPHERE FROM THE CARBURIZING ZONE ANATMOSPHERE WHICH, WHEN MIXED WITH ANY ATMOSPHERE FLOWING INTO THEDIFFUSING ZONE FROM THE CARBURIZING ZONE, PROVIDES THE ATMOSPHEREREQUIRED THEREIN, AND WITHDRAWING ATMOSPHERE FROM THE ONE OF THECARBURIZING AND DIFFUSING ZONES TOWARD WHICH ATMOSPHERE FLOWS FROM THEOTHER AT SUBSTANTIALLY THE RATE AT WHICH ATMOSPHERE IS INTRODUCED INTOTHE SEVERAL ZONES.